The invention relates to a process for isolating one or more organic substances from a gas mixture in which these organic substances are present, in which the gas mixture is subjected to quenching, and also to an apparatus for carrying out the process.
Many chemical production processes result in a reaction gas which comprises the desired reaction product and also further substances such as unreacted starting materials and undesired by-products. Since a considerable quantity of heat of reaction may be liberated in production processes from which the reaction gases are obtained, such reaction gases frequently have a high temperature. In industry, the reaction gases are usually cooled by use of heat exchangers. A fundamental distinction is made between xe2x80x9cdirectxe2x80x9d and xe2x80x9cindirectxe2x80x9d heat exchange. In direct heat exchange, the stream is brought into direct contact with the heating or cooling medium.
An important example of such a direct cooling process is the quenching of pyrolysis gases by spraying oil directly into them. However, quenching is also employed for cooling hot reaction gases from the industrial production of acrylonitrile or methacrylonitrile by the Sohio process. The Sohio process involves the reaction of propylene or isobutene, oxygen and ammonia in one step over bismuth/molybdenum catalysts in a fluidized bed. The synthesis itself proceeds with liberation of a considerable quantity of heat of reaction, so that the reaction gases leaving the acrylonitrile or methacrylonitrile reactors have a very high temperature. In the quenching procedure, the hot reaction gas is then brought into direct contact with the liquid cooling medium, usually water. In this procedure, small amounts of organic gases are washed from the reaction gas in the quenching column.
Quenching apparatuses are known in a wide variety of designs. Thus, the reaction gas to be cooled and the cooling liquid can be conveyed either in cocurrent or in countercurrent.
The isolation of acrylonitrile or methacrylonitrile from the reaction gases and, in particular, achievement of an increase in the product yield is of high importance in industry.
U.S. Pat. No. 4,720,566 discloses, for example, the addition of a hydroxylamine and a phenylenediamine to the reaction gases of an acrylonitrile synthesis before they enter the quenching column so as to avoid undesirable polymerization of acrylonitrile in the quenching column and thus to increase the acrylonitrile yield.
U.S. Pat. No. 5,703,268 describes firstly quenching the reaction gases of an acrylonitrile or methacrylonitrile synthesis with water and subsequently passing the resulting cooled gas stream containing acrylonitrile or methacrylonitrile to an absorber column where the acrylonitrile or methacrylonitrile is brought into contact with water in countercurrent and absorbed by the water. To separate off the acrylonitrile or methacrylonitrile, the aqueous solution of acrylonitrile or methacrylonitrile is then passed through a first distillation column (recovery column) and the bottom product is subsequently passed through a second distillation column (stripper column). The process improvement of U.S. Pat. No. 5,703,268 comprises increasing the hydraulic capacity of the apparatuses by means of specific pressure increases in the two distillation columns.
U.S. Pat. No. 4,234,510 describes feeding the reaction gas which has been cooled in the quenching column firstly into a column provided with cooling elements in which a certain amount of acrylonitrile or methacrylonitrile is then condensed before the gas is introduced into the absorber column.
Furthermore, U.S. Pat. No. 3,936,360 discloses recirculating the bottom product obtained in the final distillation columns to the quenching column and utilizing it there as at least part of the quenching liquid.
The quenching apparatuses described in the abovementioned processes for cooling hot reaction gases have a disadvantage common to all of them. The quenching apparatuses are customarily constructed so that the quenching liquid is circulated and a bottom volume is present below the mixing zone of reaction gas and quenching liquid. An amount of quenching liquid which is sufficiently large for operation of the quenching liquid circulation pump is held in this bottom volume. This frequently results in long residence times of the quenching liquid in the bottom volume and in the pump circuit. However, such long residence times constitute a considerable disadvantage since undesirable reactions then occur between the components dissolved in the quenching liquid.
In the case of the acrylonitrile synthesis, a certain proportion of the acrylonitrile produced and of the ammonia is dissolved in the quenching liquid from the hot reaction gases during quenching. The acrylonitrile and ammonia present in the liquid then react with one another to form undesirable by-products, for example hydrolysis products of acrylonitrile and oligomers of acrylonitrile, sometimes also with ammonia which can be described by the following simplified structural formula: [xe2x80x94NH2xe2x80x94CH2CH2(CN)xe2x80x94]x. In this way, an amount of up to 3% of the acrylonitrile previously produced is lost again.
U.S. Pat. No. 3,876,508 discloses passing the reaction gases of an acrylonitrile synthesis into a quenching apparatus in which an aqueous solution of a mineral acid is used as quenching liquid. This leads to the ammonia dissolved in the quenching liquid being immediately neutralized to form corresponding ammonium salts and therefore not being able to react with likewise dissolved acrylonitrile to form undesirable by-products. However, the quenching liquid still has to be worked up, firstly to separate off the ammonium salts present therein and, secondly, to recover the approximately 3% of dissolved acrylonitrile. For this purpose, the quenching liquid is fed into a subsequent distillation or stripping column where, for successful distillation or stripping, it is absolutely necessary for the pH of the quenching liquid to be held at a value of not more than 5, preferably not more than 3, by addition of an acid. Only by means of this measure is it possible to suppress undesirable subsequent or polymerization reactions of the still dissolved acrylonitrile. Overall, this variant is complicated in terms of apparatus.
It is therefore an object of the present invention to provide a process by means of which organic substances, in particular acrylonitrile or methacrylonitrile, can be recovered and isolated without use of complicated apparatus and at the same time in high yield from reaction gases in which these substances, in particular acrylonitrile or methacrylonitrile, are present.
The invention accordingly provides a process for isolating one or more organic substances from a gas mixture in which these organic substances are present, in which the gas mixture is subjected to a quenching in a column, characterized in that quenching is carried out in the upper part of the column and the quenching liquid is subjected to stripping in the lower part of the column.
In the process of the present invention, the gas mixture introduced is quenched in the upper part of the column by bringing it into intimate contact with a stream of quenching liquid. This results in rapid cooling of the gas mixture fed in. The stream of quenching liquid is subsequently subjected to stripping in the lower part of the column by bringing the quenching liquid into contact with an inert gas. As a result, the organic substances dissolved in the quenching liquid are carried from the column with the inert gas.
In the process of the invention, preference is given to using a gas mixture from an acrylonitrile or methacrylonitrile synthesis.
The invention further provides a column for carrying out the process of the invention, in which both quenching and stripping can be carried out and which is characterized in that
a) it has an upper quenching section and a lower stripping section,
b) it has a lateral feed facility for a reaction gas stream below the quenching section and, above the quenching section, a further lateral feed line for the quenching liquid,
c) it has, above the quenching section, a liquid distributor through which the quenching liquid is introduced into the column,
d) a feed line for the stripping gas is located below the stripping section and is arranged either above the bottom volume of the column or else at the height of the bottom volume,
e) the bottom volume of the column is connected to the liquid distributor of the quenching section via a pump-driven feed line and
f) optionally, part of the bottom volume can be discharged from the column via a lateral discharge line,
g) a feed line for fresh quenching liquid is present at the height of the bottom volume and
h) a discharge line for discharge of the product gas stream is located at the top of the column.